1. Field of the Invention
The present invention relates to an automatic transmission control system having a fail-safe function, and specifically to hydraulic technologies for a hydraulic control system of an electronically-controlled automatic transmission capable of electronically controlling an engaging pressure of each of engaging elements or controlling elements such as clutches and brake bands directly by means of a transmission ECU (electronic control unit), and of providing simplified hydraulic circuits, reduced hydraulic component parts, small-sized valve bodies, and expanded design flexibility, and lighter hydraulic packages.
2. Description of the Prior Art
In recent years, there have been proposed and developed various electronically-controlled automatic transmissions of simplified hydraulic circuits, reduced hydraulic component parts, and small-sized valve bodies. One such electronically-controlled transmission (which will be hereinafter abbreviated to an xe2x80x9cECT transmissionxe2x80x9d) has been disclosed in Japanese Patent Provisional Publication No. 8-121586. The ECT transmission disclosed in the Japanese Patent Provisional Publication No. 8-121586, has a low-and-reverse brake (LandR/B) pressure control device through which a low-and-reverse brake (LandR/B) is applied or engaged in a drive range and first gear, and released or disengaged in a drive range and either of second, third and fourth (overdrive) gears. In more detail, as shown in FIG. 9, the above-mentioned low-and-reverse brake (LandR/B) pressure control device uses two different hydraulic pressures, namely one being a second brake pressure P2ND which is applied to a second brake for the purpose of applying the second brake in a drive range and second gear or in a drive range (D range) and fourth gear (OD gear), and the other being an overdrive clutch pressure POD which is applied to an overdrive clutch for the purpose of engaging the overdrive clutch in a drive range and third gear or in a drive range and fourth gear. The low-and-reverse brake (LandR/B) pressure control device functions to forcibly drain the pressure supply line of the low-and-reverse brake in the D range and 2nd gear, in the D range and 3rd gear, or in the D range and 4th gear, in which at least one of the second brake pressure P2ND and the overdrive clutch pressure POD are created or produced.
In the hydraulic control device of the electronically-controlled automatic transmission disclosed in the Japanese Patent Provisional Publication No. 8-121586, however, there is no means for detecting as to whether a first spool 51 is sticking in its valve housing. Assuming that a failure in a hydraulic system for the low-and-reverse brake (LandR/B) occurs under a particular condition where the first spool 51 is sticking in the left-hand side position (viewing FIG. 9), the low-and-reverse brake pressure may be created and often held at a high pressure level in the hydraulic system for the low-and-reverse brake (LandR/B). As a result of this, the automatic transmission will fall into a so-called interlocking state in which the low-and-reverse brake and the second brake are both applied or engaged.
Accordingly, it is an object of the invention to provide an automatic transmission control system having a fail-safe function, which avoids the aforementioned disadvantages of the prior art.
It is another object of the invention to provide an automatic transmission control system for an electronically-controlled automatic transmission, which has a fail-safe function capable of providing an optimal timing of a forcible pressure-release of an engaging-element pressure (simply, an engaging pressure) applied to an engaging element or an optimal timing of a forcible oil drain from the engaging element, without exerting a bad influence upon an automatic shifting control and without providing an undesired automatic transmission interlock in presence of a hydraulic system failure.
It is a still further object of the invention to provide a transmission control system for an electronically-controlled automatic transmission, which ensures a reliable hydraulic system failure detection.
In order to accomplish the aforementioned and other objects of the present invention, an automatic transmission control system having a fail-safe function for an automatic transmission, comprises sensor means for detecting at least a throttle opening and a vehicle speed, a gear mode determination means for determining a current gear mode based on the throttle opening and the vehicle speed, an engagement-disengagement command means for outputting a command based on the current gear mode determined by the gear mode determination means and corresponding to engagements and disengagements for a plurality of engaging elements to an actuator which operates the engaging elements, a transmission gear train which performs a multi-step speed-change by a combination of engagements and disengagements of the engaging elements, a set ON-OFF state storage means for estimating a normal state of engagements and disengagements of the engaging elements based on the current gear mode, and for storing the normal state of engagements and disengagements of the engaging elements, an actual ON-OFF state detection means for detecting an actual state of engagements and disengagements of the engaging elements, an actual ON-OFF state storage means for storing the actual state of engagements and disengagements of the engaging elements, a comparison-and-determination means for comparing the normal state of engagements and disengagements of the engaging elements, stored in the set ON-OFF state storage means, with the actual state of engagements and disengagements of the engaging elements, stored in the actual ON-OFF state storage means, and for determining that a hydraulic system failure occurs when the actual state detected and the normal state estimated disagree with each other, and a fail-safe mode engagement-disengagement command means for selecting a specified gear mode based on the actual state of engagements and disengagements of the engaging elements, stored in the actual ON-OFF state storage means, and for outputting a command corresponding to engagements and disengagements of the engaging elements to the actuator so that the automatic transmission is shifted to the specified gear mode which is preprogrammed to be suitable for the actual state, only when the comparison-and-determination means determines that the hydraulic system failure occurs. It is preferable that the comparison-and-determination means has a hydraulic system failure counter whose counted value is incremented once each time a state that the actual state detected disagrees with the normal state estimated occurs continuously for a predetermined time period. The comparison-and-determination means determines that the hydraulic system failure occurs only when the counted value of the hydraulic system failure counter reaches a predetermined value. More preferably, the specified gear mode selected by the fail-safe mode engagement-disengagement command means may be preprogrammed to remain at a gear mode determined by the gear mode determination means just before detecting the hydraulic system failure, or to be up-shifted with respect to the gear mode determined by the gear mode determination means just before detecting the hydraulic system failure, or to shift the automatic transmission to neutral. It is preferable that the output of the command corresponding to the specified gear mode selected by the fail-safe mode engagement-disengagement command means may be limited to only a time period from a time when the hydraulic system failure is detected to a time when the vehicle speed becomes less than a predetermined low speed value. More preferably, the actual ON-OFF state detection means may comprise an oil pressure switch provided for detecting a hydraulic pressure in a hydraulic system containing each of the engaging elements. Furthermore, the comparison-and-determination means may comprise an inhibition means inhibiting a diagnosis upon a hydraulic system failure during shifting or during transmission range gear selection during which there is a transition of at least one of the engaging elements from one of engaged and disengaged states to the other.